As means for transporting products or transmitting a drive power from one place to another, there has been well known a belt mechanism which includes an endless belt rotatably supported by a plurality of rollers, at least one of which being drivingly connected with a motor for rotating the endless belt. Among other fields, this belt mechanism has been employed in an image forming apparatus, such as copier, facsimile, and printer.
Japanese Laid-Open Publication No.63-313182 discloses an image forming apparatus including a belt fixing device for fusing an unfused toner image and then fixing it to a recording sheet. The belt-type fixing device has a drive roller, a driven roller, and an endless fixing belt made from thin walled film and entrained around the rollers, a heater disposed inside the fixing belt, and a pressure roller arranged outside the belt to nip a portion of the belt with the heater.
This belt type fixing device, however, has a drawback that the belt inevitably moves, or steers, in its transverse direction due to errors in circularities, surface configurations, and degree of parallelization of the rollers and a non-uniformity of thickness of the belt. This possibly causes the fixing belt to be damaged unless the transverse movement has been corrected. To overcome this problem, there have been proposed a device for detecting the transverse movement of the belt using a charge coupled device (CCD) or other optical sensor consisting of a pair of a light emitter and a light receiver and a device for correcting the transverse movement.
The optical detecting device occupies a relatively large space and is easy to be adversely affected by environmental variations. Also, a detecting component, e.g., lens, of the optical sensor can be soiled by dust generated both from products to be transported by the belt and from the belt itself, which possibly results in a false detection of the transverse movement of the belt. Further, the above mentioned belt fixing device having the heater requires the optical sensor to be arranged apart from the heater, limiting an arrangement of the optical detecting device. A thermal affection of the heater can be reduced to a certain extent by spacing the sensor from the heater, however, it is impossible to remove the thermal affection completely and therefore there still remains possibilities that reduce a duration of the sensor and result in the false detection of the transverse movement.